The present invention generally relates to atmospheric pressure chemical ionization (APCI) in preparation for mass analysis, as is performed in mass spectrometry (MS). More particularly, the present invention relates to an apparatus and method for improving ionization of sample molecules in an APCI source.
Mass spectrometry is a highly sensitive method of molecular analysis. In general, mass spectrometry is a technique that produces a mass spectrum by converting the components of a sample into rapidly moving gaseous ions, and resolving the ions on the basis of their mass-to-charge (m/e or m/z) ratios. The mass spectrum can be expressed as a plot of relative abundances of charged components as a function of mass, and thus can be used to characterize a population of ions based on their mass distribution. Mass spectrometry is often performed to determine molecular weight, molecular formula, structural identification, and the presence of isotopes. The apparatus provided for implementing mass spectrometry, i.e., a mass spectrometer (MS) system, typically consists of a sample inlet system, an ion source, a mass analyzer, and an ion detection system, as well as the components necessary for carrying out signal processing and readout tasks. Many of these functional components of the mass spectrometer, particularly the mass analyzer, are maintained at a low pressure by means of a vacuum system. The ion source converts the components of a sample into charged particles. The negative particles are ordinarily removed from the process flow in positive ion mode when analyzing positive particles. In negative ion mode the positive ions are removed. The mass analyzer disperses the charged particles based on their respective masses, and then focuses the ions on the detector. The ion currents produced by the detector are then amplified and recorded as a function of spectral scan time. The designs of the components of the mass spectrometer, and the principles by which they operate, can vary considerably. Thus, components of differing designs have distinct advantages and disadvantages when compared to each other, and the desirability of any one design can depend on, among other factors, the nature of the sample to be analyzed.
The sample inlet system employed for mass spectrometry can be chromatographic. That is, the effluent from a chromatographic column can be utilized as the sample source for the MS system. The mass spectrometer in such cases can be considered as serving as the detector for the chromatographic apparatus. Such an arrangement is commercially available in systems in which a gas chromatographic (GC) apparatus is directly coupled to the mass spectrometer (GC/MS systems), or a liquid chromatographic (LC) apparatus is directly coupled to the mass spectrometer (LC/MS systems). These combined systems are particularly useful for deriving complex spectra from mixtures, as it is known that mass spectrometers alone are more or less limited to handling pure compounds and relatively simple mixtures.
An ion source commonly serving as the interface between an LC apparatus and the mass spectrometer operates according to the principle of atmospheric pressure ionization (API), a soft ionization technique in which ionization of a sample occurs outside of the vacuum region or regions associated with the mass spectrometer. An increasingly popular type of API technique is atmospheric pressure chemical ionization (APCI or APcI). Simply stated, APCI is a means for ionizing samples (e.g., analyte molecules) dissolved in a liquid (e.g., an excess of mobile-phase molecules such as solvent). The sample-containing liquid emitted from the LC apparatus is pneumatically nebulized into a fine dispersion of numerous small droplets, typically below 100 microns in diameter. Heat is applied to the droplets to vaporize the liquid and sample matrix. This nebulization/vaporization process, however, is gentle enough to preserve the molecular identity of the sample constituents at this stage. The resulting gas/vapor is subsequently passed into a chamber where electrons emitted from an electrode generate a low-current corona discharge in the ambient, atmospheric-pressure environment consisting of, for example, a background gas such as nitrogen or air. The corona discharge ionizes the mobile-phase molecules to form an energetic chemical reagent gas plasma. In the corona discharge, ion-molecule reactions occur between the charge-neutral sample and the reagent ions formed in the primary discharge. The dominant mechanisms for the ion-molecule reactions are collisions between the reagent ions and the sample molecules, enabled by the relatively high (atmospheric) pressure environment, and charge transfer reactions. The ion-molecule reactions cause the sample to become charged, and the resulting stable sample ions are passed through an opening in a vacuum chamber into the mass analyzer of the mass spectrometer for mass analysis. Unlike the API technique of electrospray ionization (ES), in which multiple-charged molecular ions [M+nH]n+ are produced, in most applications APCI produces only single-charged molecular ions typically in the form of [M+H]+ or [Mxe2x88x92H]xe2x88x92 as a result of protonation or deprotonation.
FIG. 1 illustrates an example of a conventional APCI source, generally designated 10, utilized in, for example, an LC/MS system. In general terms, APCI source 10 comprises a sample introduction and nebulizing section, generally designated 20; a vaporization section, generally designated 30; an ionization section, generally designated 40; and an ion inlet section, generally designated 50. Ion inlet section 50 includes a front plate 52 having an ion inlet aperture 53 through which ionized products are directed into the mass analyzer of the mass spectrometer. For simplicity, the mass analyzer and other typical components of the mass spectrometer, such as its ion detection, signal processing and readout systems, are collectively designated as MS in FIG. 1.
Nebulizing section 20 comprises a capillary tube 23, typically a metal capillary, that serves as the sample inlet system of mass spectrometer MS. Capillary tube 23 conducts the LC column flow from a liquid chromatographic apparatus LC into vaporization section 30. In addition, a length of conduit 27 for directing a suitable inert nebulizing gas such as nitrogen into vaporization section 30 is coaxially disposed about capillary tube 23. Vaporization section 30 of APCI source 10 generally includes a vaporizing tube 33 and a heater 35 enclosed in a coaxial housing (not shown), and a conduit 37 for directing a suitable inert vaporizing (xe2x80x9cauxiliaryxe2x80x9d or xe2x80x9cmake-upxe2x80x9d) gas such as nitrogen into vaporizing tube 33. Heater 35 is situated so as to ensure sufficient thermal contact with the wall of vaporizing tube 33. The wall of vaporizing tube 33 is typically quartz, and can operate at temperatures ranging from about 200-600xc2x0 C. to rapidly vaporize effluent from capillary tube 23. While the thermal effect on typical samples is minimal, such a technique is not compatible with very thermally labile molecules. Capillary tube 23 is disposed along the central axis of vaporizing tube 33 and terminates at a capillary tube outlet 23A within vaporizing tube 33. A portion of vaporizing gas conduit 37 is coaxially disposed about nebulizing gas conduit 27 as well as capillary tube 23.
Ionization section 40 of APCI source 10 generally includes an ionization chamber 42 defining an enclosed volume into which a corona needle or pin 43 is inserted. Capillary tube 23 and conduits 27 and 37 are often integrated in a manifold structure which, along with vaporization section 30, is often structured as a probe that is mounted to ionization chamber 42. Corona needle 43 typically operates at about 5-10 kV and 1-5 mA to strike a low-current corona discharge or electron cloud 45 within ionization section 40. This electrical discharge 45 enables the generation of the afore-mentioned chemical reagent gas plasma utilized to ionize the sample molecules. Vaporizing tube 33 terminates at a vaporizing tube outlet 33A in fluid communication with ionization chamber 42, whereby vaporized analyte and mobile-phase constituents are transferred into chamber 42 for ionization. One or more voltage sources (not shown) are typically provided to impress a voltage between front plate 52 of ion inlet section 50 and one or more electrically conductive surfaces in ionization section 40 such as corona needle 43, thereby establishing one or more electric fields sufficient to attract ionized products derived from the vaporized LC eluent into ion inlet section 50 through ion inlet aperture 53.
In operation, a liquid sample comprising the LC column flow from liquid chromatographic apparatus LC is introduced into the heated vaporizing tube 33 via capillary tube 23, typically at a flow rate of about 0.1-2.0 ml/min. Nebulizing and vaporizing gas streams are introduced into vaporizing tube 33 through nebulizing gas conduit 27 and vaporizing gas conduit 37, respectively. The nebulizing gas flows concentrically around centrally disposed capillary tube 23 at a high velocity and a typical pressure of about 0.8 MPa, thereby nebulizing the liquid sample into small liquid droplets as the nebulizing gas and liquid sample enter vaporizing tube 33. Because the wall of vaporizing tube 33 is heated by heater 35 and consequently transfers heat energy into the interior of vaporizing tube 33, the liquid droplets of the nebulized sample entering vaporizing tube 33 are converted into vapor. The vaporizing gas is added to the system at a typical flow rate of about 1-3 L/min by means of vaporizing gas conduit 37. The flow of vaporizing gas assists in sweeping or transporting the liquid-droplet and vapor phases of the sample-containing aerosol through vaporizing tube 33. The resulting vapor temperature of the aerosol is about 100xc2x0 C. The heated gas/vapor then passes in a sample exhaust stream 60 into chamber 42 and into the low-current corona discharge 45 established by corona needle 43 in ionization section 40, where the charge-neutral sample is ionized by ion-molecule reactions with the reagent ions formed in corona discharge 45.
In a typical configuration of conventional APCI source 10, corona needle 43 is oriented toward and in relatively close proximity with ion inlet aperture 53. Accordingly, a relatively large space or gap exists between vaporizing tube outlet 33A and the ionization volume defined by corona discharge 45. Moreover, corona discharge 45 is typically established by electrically coupling corona needle 43 with front plate 52 of ion entry section 50. As a result, vapor flow in ionization chamber 42 is characterized by an undesirably large volumetric time constant, which in turn results in a large-volume mixture of vapor-phase sample and vapor-phase background species (i.e., non-sample constituents). This mixture leads to an increase in the formation of background ions and in the splitting of peak components of the sample, a concomitant reduction in reaction volume and thus a reduction in sample ions, and an increase in chemical noise (i.e., a reduction in signal-noise ratio) and peak tailing or broadening as generated by mass spectrometer MS. In addition, corona needle 43 extends into sample exhaust 60 and thus is subject to contamination, especially at high flow rates.
It would therefore be advantageous to provide an ion source and ionization method that minimizes the amount of background vapor mixing with sample vapor, increases reaction volume, reduces the number of background ions entering a mass spectrometer, and reduces sample peak tailing. It would be further advantageous to-provide an ion source in which the electrode or electrodes employed are not directly exposed to the vaporizer discharge and to the chemical environment of the source chamber into which the contents of the vaporizing tube are exhausted.
The present invention is provided to address, in whole or in part, these and other problems associated with the prior art.
In general terms, the present invention provides an apparatus and method for ionizing a sample in preparation for mass analysis. The sample is first nebulized by pneumatic means and then vaporized by heating means. The vaporized sample is then ionized by directing the sample through an electrical discharge. The ionized sample is then directed toward the inlet section of an appropriate mass analysis device such as a mass spectrometer. The electrical discharge is formed at a location within the apparatus that enables the sample to be ionized without any significant mixing with background gases or vapors, and thus background noise and peak tailing are avoided or reduced during mass analysis. In some embodiments the electrical discharge has a DC potential, while in other embodiments the electrical discharge has an AC potential.
According to one embodiment, an apparatus for use as an ion source for mass analysis comprises a nebulizing device for nebulizing a flowing sample, a vaporizing device for vaporizing the sample flowing from the nebulizing device, a chamber, an ion sampling structure, and an ionizing device. The vaporizing device comprises a vaporizing interior that terminates at a vaporizing device outlet. The chamber fluidly communicates with the vaporizing device outlet. The ion sampling structure has an ion sampling inlet that fluidly communicates with the chamber and is spaced from the vaporizing device outlet. The ionizing device comprises first and second electrodes. The electrodes are positioned so as to produce an electrical discharge therebetween at a location closer to the vaporizing device outlet than to the ion sampling inlet.
In one aspect of this embodiment, the first electrode is positioned in the chamber in close proximity to the vaporizing device outlet. The second electrode is disposed within the vaporizing interior such that an electrical discharge is produced that extends into the vaporizing interior through the vaporizing device outlet. The second electrode can be a point-charge device such as a needle or pin, or can take the form of an electrically conductive portion of the nebulizing device or the vaporizing device. Alternatively, the second electrode is positioned in the chamber in the close proximity to the vaporizing device outlet opposite to the first electrode, such that the electrical discharge traverses a sample exhaust flow from the vaporizing device outlet. As another alternative, the first and second electrodes are disposed along an axial length of the vaporizing device outside of the vaporizing interior and are coupled by an AC voltage to produce an electrical discharge substantially entirely within the vaporizing interior. Preferably, the AC voltage is a high frequency voltage such as an RF voltage.
According to any of the embodiments described herein, the components of the apparatus serving as electrodes are positioned so as not to contact the sample in order to prevent contamination of the electrodes.
According to another embodiment, an apparatus for use as an ion source for mass analysis comprises a nebulizing device, a vaporizing device, a chamber, an ion sampling structure, and an ionizing device. The ionizing device comprises an electrode disposed in the chamber for creating an electrical discharge between the electrode and an electrically conductive component disposed in an interior of the vaporizing device. In one aspect, a DC voltage source is connected between the electrode and the conductive portion. In another aspect, an AC voltage source is connected between the electrode and the conductive portion.
According to yet another embodiment, an apparatus for use as an ion source or mass analysis comprises a nebulizing device, a vaporizing device, a chamber, an ion sampling structure, and a ionizing device. The ionizing device comprises first and second electrodes disposed in the chamber for creating an electrical discharge therebetween, across a sample exhaust flow received in the chamber from the vaporizing device, and proximal to an outlet of the vaporizing device into the chamber.
In one aspect of this embodiment, an RF voltage source is connected between the first and second electrodes. In another aspect, in addition to the RF voltage source, a DC voltage source is connected between one or both of the electrodes and the ion sampling structure for establishing an electrical field for directing sample ions toward an inlet of the ion sampling structure.
According to another embodiment, an apparatus for use as an ion source for mass analysis comprises a nebulizing device, a vaporizing device, a chamber, an ion sampling structure, and an ionizing device. The ionizing device comprises a first electrode disposed in the chamber approximate to an outlet of the vaporizing device, and a second electrode disposed in an interior of the vaporizing device. The configuration of these electrodes creates an electrical discharge through the vaporizing device outlet and into the interior of the vaporizing device.
According to another embodiment, an apparatus for use as an ion source for mass analysis comprises a nebulizing device, a vaporizing device, a chamber, an ion sampling structure, and an ionizing device. The ionizing device comprises a first and second electrodes that are driven by a RF voltage. The first and second electrodes are disposed outside of the interior of the vaporizing device along a length of the vaporizing device for creating an electrical discharge substantially entirely within the interior. In one aspect of this embodiment, an additional, polarizing electrode is disposed in the chamber for establishing an electrical field by which ionized sample components can be directed toward an inlet of the ion sampling structure.
A method is provided for ionizing sample molecules at atmospheric pressure, comprising the following steps. A nebulized sample is flowed through an interior of a vaporizing device to vaporize the sample. The vaporized sample is exhausted through an outlet of the vaporizing device into a chamber. An ion sampling inlet is disposed in the chamber and is spaced from the vaporizing device outlet. The sample is ionized by forming an electrical discharge at a location that is closer to the vaporizing device outlet than to the ion sampling inlet.
In one aspect of this method, at least a portion of the electrical discharge is directed through the vaporizing device outlet into the vaporizing device interior to initiate ionizing reactions prior to the sample being exhausted into the chamber. In another aspect, the sample is exhausted into the chamber in a sample exhaust stream and the electrical discharge traverses the sample exhaust stream at a location immediately downstream from the vaporizing device outlet. The sample becomes ionized immediately after being exhausted from the vaporizing device outlet. In a further aspect, the electrical discharge is formed substantially and entirely within the vaporizing device interior to initiate ionizing reactions prior to the sample being exhausted into the chamber.